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Tuesday, January 27, 2009

When the demand for higher-octane gasoline developed during the early 1930s, attention was directed to improving the octane number of fractions within the boiling range of gasoline. Straight-run (distilled) gasoline frequently had very low octane numbers, and any process that would improve the octane numbers would aid in meeting the demand for higher-octane-number gasoline. Such a process (thermal reforming) was developed and used widely, but to a much lesser extent than thermal cracking. Thermal reforming converts (reforms) gasoline into higher-octane gasoline. The equipment for thermal reforming is essentially the same as for thermal cracking, but uses higher temperatures.

Thermal reforming

In a thermal reforming process, the feedstock, such as 205°C (400°F) end-point naphtha or a straight-run gasoline, is heated to 510-595°C (950-1100°F) in a furnace, much the same as a cracking furnace, with pressures from 400 to 1000 psi (2758 to 6895 kPa). As the heated naphtha leaves the furnace, it is cooled or quenched by the addition of cold naphtha and then enters a fractional distillation tower where the heavy products are separated. The remainder of the reformed material leaves the top of the tower and is separated into gases and reformate. The reformate has a high octane number due to cracking of the longer-chain paraffins into higher-octane olefins.

The products of thermal reforming are gases, gasoline, and residual oil or tar, the latter being formed in very small amounts (about 1%). The amount and quality of gasoline, known as reformate, is very dependent on the temperature. As a rule, the higher the reforming temperature, the higher the octane number, but the lower the yield of reformate.

Catalytic reforming

Like thermal reforming, catalytic reforming converts low-octane gasoline into high-octane gasoline (reformate). While thermal reforming could produce reformate with octane numbers of 65-80 depending on the yield, catalytic reforming produces reformate with octane numbers on the order of 90-95.

Catalytic reforming usually is carried out by feeding a naphtha (after pretreating with hydrogen if necessary) and hydrogen mixture to a furnace for heating to the desired temperature, 450-520°C (840-965°F), and then passed through fixed-bed catalytic reactors at hydrogen pressures of 100-1000 psi (689-6895 kPa). Normally pairs of reactors are used in series, with heaters located between adjoining reactors to compensate for the endothermic reactions taking place. Sometimes as many as five reactors are kept on stream in series while one or more is being regenerated. The on-stream cycle of any one reactor may vary from several hours to many days, depending on the feedstock and reaction conditions.

The commercial processes available for use are broadly classified as the fixed-bed, moving-bed, and fluid-bed types. Fixed-bed processes use predominantly platinum-containing catalysts in units equipped for cycle, occasional, or no regeneration, whereas the fluid- and moving-bed processes use mixed nonprecious metal oxide catalysts in units equipped with separate regeneration facilities.

Dehydrogenation is a major chemical reaction in catalytic reforming, producing large quantities of hydrogen gas. The hydrogen is recycled though the reforming reactors, providing the atmosphere necessary for the chemical reactions and preventing carbon deposition on the catalyst, thus extending its operating life. An excess of hydrogen, above whatever is consumed in the process, is produced. As a result, catalytic reforming processes are unique in that they are the only petroleum refinery processes to produce hydrogen as a by-product. See also: Dehydrogenation

The reforming catalyst composition is dictated by the composition of the feedstock and the desired reformate. The catalysts used are principally molybdena-alumina (Mo2O3-Al2O3), chromia-alumina (Cr2O3-Al2O3), or platinum on a silica-alumina (Pt-SiO2-Al2O3) or alumina (Pt-Al2O3) base. Nonplatinum catalysts are widely used in regenerative process for feeds containing sulfur, which poisons platinum catalysts, although pretreatment processes (hydrodesulfurization) may permit platinum catalysts to be used.

The purpose of platinum on the catalyst is to promote dehydrogenation and hydrogenation reactions, that is, the production of aromatics, participation in hydrocracking, and rapid hydrogenation of carbon-forming precursors. For the catalyst to have an activity for isomerization of both paraffins and naphthenes – the initial cracking step of hydrocracking – and to participate in paraffin dehydrocyclization, it must have an acid activity. The balance between these two activities is most important in a reforming catalyst. In fact, in the production of aromatics from cyclic saturated materials (naphthenes), it is important that hydrocracking be minimized to avoid loss of the desired product; and, thus, the catalytic activity must be moderated relative to the case of gasoline production from a paraffinic feed, where dehydrocyclization and hydrocracking play an important role.